Abstract
The Scannning Electron Microscope operated in the back-scattered electron mode has been applied in a microstructural analysis of chromites. Samples from the Oman, Vourinos (Greece) and Tiebaghi (New Caledonia) ophiolites, displaying a variety of mantle flow and emplacement structures, have been examined and compared with undeformed chromite from the Bushveld intrusion and deformed chromite from the high-grade metamorphic Fiskenaesset complex. Channelling contrast and electron channelling patterns provide information on grain boundaries, subgrains, inclusions and cracks, as well as defects in the chromite lattice. These microstructures suggest that dislocation mechanisms are active in chromite deforming under mantle conditions. Recovery is quick whereas nucleation is slow and, accordingly, recrystallization of chromite occurs by migration of grain boundaries, driven either by strain energy or by interfacial energy, increasing the grain size during deformation. Dislocation mechanisms, however, only provide a secondary mode of deformation compared to brittle fracturing and passive rotation of chromite grains and fragments in the weaker (silicate) matrix. Diffusion-accommodated flow may also be significant.
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